1. Field of the Invention
The present invention relates to a fingerprint input device and particularly relates to a fingerprint input device for directly obtaining a fingerprint image using a two-dimensional image sensor.
2. Description of Related Art
Conventionally, a fingerprint input device of this type normally employs optical components such as a lens, a prism and an optical fiber, irradiates light to the fingerprint of a fingertip from a predetermined angle, converges light reflected by the fingerprint and outputs a fingerprint image. Further, a system of directly inputting a fingerprint image by electrostatic capacity instead of obtaining a fingerprint image using an optical system is studied.
A system of directly inputting a fingerprint image by an optical device is also studied. As the system, there are known a light reflection system for irradiating light to a fingertip skin and using reflected light therefrom and a light transmission system for inputting light into a finger and receiving the light emitted from the finger surface on the end face of an optical fiber flux.
According to these conventional fingerprint input devices, however, if optical components such as a lens, a prism or an optical fiber is employed, it is necessary to secure the magnitude of the optical components themselves, attachment locations therefor and the like to some extent. In addition, the optical components are relatively expensive, which disadvantageously prevents the fingerprint input device from being made thin and small in size and even prevents providing the device at low cost. Besides, if a fingerprint image is optically processed by the optical components, the obtained image disadvantageously tends to be distorted.
On the other hand, the system for directly inputting a fingerprint image by electrostatic capacity is disadvantageous in that the device is very weak to external static electricity and it is difficult to obtain good sensitivity.
Moreover, there is conventionally known an optical, reflection type fingerprint input device for directly obtaining a fingerprint image. According to this fingerprint input device, light is applied to a finger which faces the right surface of a two-dimensional image sensor from the rear surface of the sensor through the gap in the sensor. In a fingerprint ridgeline portion in which a film provided between the two-dimensional image sensor and the finger contacts the finger, the light applied to the finger enters the finger without being reflected by the contact surface. In a fingerprint valley portion in which the film is out of contact with the finger, the light not reflected by the film and the light emitted from the film is reflected by the surface of the finger and is incident again on the two-dimensional image sensor through the film. It is, therefore, possible to discriminate the fingerprint ridgeline portion from the fingerprint valley portion by whether or not the reflected light is incident on the two-dimensional image sensor. According to this device, it is necessary to apply light to the finger which faces the front surface of the two-dimensional image sensor, from the rear surface of the sensor. As a result, many portions which let light pass therethrough are required between sensor elements, which decreases the density of the sensor elements, sacrifices the resolution of the sensor, requires a special structure for the sensor, thereby pushing up manufacturing cost. Further, this device has an operational disadvantage in that if ambient light (disturbance light) enters the finger from a portion other than the measurement target surface of the finger, and is scattered in the finger, and passed through the contact surface between the fingerprint ridgeline portion and the film and incident on the optical image sensor, then the intensity of the incident light becomes almost equal to the reflected light and a fingerprint image cannot be obtained.
Furthermore, the conventionally known transmission type fingerprint input device utilizes a phenomenon that if a finger is pressed against the end face of an optical fiber flux and light is applied to the finger, then a fingerprint image appears. Due to this, it is required to use an intact and long optical fiber flux or a short slice of the optical fiber. As a result, the fingerprint input device becomes disadvantageously large in size or the manufacturing cost of the device is disadvantageously pushed up. In addition, there is a limit to making the optical fiber quite short, i.e., slicing the optical fiber to be thin, thereby disadvantageously restricting an attempt to make the device thin. Furthermore, the need of fingerprint input devices has widely spread to various fields. As a result, it become necessary to consider a case where a device is bent like an IC card, to attach the device to a curved surface such as a grip of various types, a writing tool or a shaft such as a pen-like pointer or the like. Therefore, it is insufficient to provide only a conventional two-dimensional image sensor which uses monocrystal silicon and which is not flexible at all.
The inventor of the present invention disclosed a fingerprint input device intended to solve these problems in Japanese Patent Application Laid-Open No. 2000-217803. This fingerprint input device is a light transmission type device, and does not include an optical fiber but a two-dimensional image sensor and a cover which is arranged on the two-dimensional image sensor, which is made of a transparent member such as glass and on which a fingertip is mounted. The fingerprint input device detects a fingerprint ridgeline portion in which the cover contacts a finger as a light portion and a fingerprint valley portion in which the cover is out of contact with the finger as a dark portion. This fingerprint input device may also include the first light source which irradiates light to the tip end of the fingertip and the second light source which irradiates light to the neighborhood of the first joint of the finger. The light emitted from these light sources enters the fingertip portion and scattered in the fingertip portion, and part of the scattered light is directed toward a fingerprint region. Therefore, it is possible to enhance the contrast of a fingerprint image detected by the two-dimensional image sensor.
However, Japanese Patent Application Laid-Open No. 2000-217803 only discloses that glass can be used as the cover employed in the fingerprint input device and does not specify the refractive index of the cover material and the thickness of the cover. As a result, depending on the refractive index of the cover material, a fingerprint image is not always sufficient in contrast. Further, depending on the thickness of the cover, a fingerprint image is not always sufficient in contrast and a protection function of protecting the two-dimensional image sensor is not always sufficient.